Scientists reveal secrets behind frog transparency

WASHINGTON (AP) — Now you see them, now you don’t.

Some frogs found in South and Central America have the rare ability to turn their nearly transparent appearance on and off, researchers report Thursday in the journal Science.

During the day, these nocturnal frogs sleep by hanging under the leaves of trees. Their delicate, greenish transparent forms cast no shadows, making them nearly invisible to birds and other predators passing overhead or below.

But when northern glass frogs wake up and hop in search of insects and mates, they turn an opaque reddish-brown color.

“When they’re transparent, it’s for their safety,” said Junjie Yao, a biomedical engineer at Duke University and co-author of the study. When awake, they can actively evade predators, but when sleeping and more vulnerable, “they have adapted to stay hidden.”

Using light and ultrasound imaging technology, researchers have discovered the secret: while they sleep, frogs concentrate or “hide” nearly 90% of their red blood cells in their liver.

Because they have transparent skin and other tissues, it is the blood flowing through their bodies that would otherwise give them away. Frogs also shrink and gather most of their internal organs, Yao said.

The research “explains beautifully” how “glass frogs conceal blood in the liver to maintain transparency,” said Juan Manuel Guayasamin, a frog biologist at San Francisco University in Quito, Ecuador, who did not participated in the study.

Exactly how they do this, and why it doesn’t kill them, remains a mystery. For most animals, having very little oxygen circulating in the blood for several hours would be lethal. And concentrating the blood so tightly would lead to deadly clotting. But somehow the frogs survive.

Further research on the species could provide useful clues for the development of anticoagulant drugs, said Carlos Taboada, a biologist at Duke University and co-author of the study.

Only a few animals, mostly ocean dwellers, are naturally transparent, said Oxford University biologist Richard White, who was not involved in the study. “Transparency is extremely rare in nature, and in terrestrial animals it is essentially unknown outside of the glass frog,” White said.

Those that are transparent include certain fish, shrimp, jellyfish, worms, and insects, none of which move large amounts of red blood through its body. The trick of hiding blood while sleeping seems to be unique to frogs.

“It’s just this really amazing, dynamic form of camouflage,” White said.

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The Associated Press Health and Science Department is supported by the Howard Hughes Medical Institute Science and Education Media Group. The AP is solely responsible for all content.

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